Tapping and holding an image in a comment sometimes brings up a share menu where you can copy the image link, etc. and sometimes it just collapses the comment. From doing some testing, I have found that whenever the image has alt text in the markdown is when it just collapses. So...

...an image in a comment written this way will bring up a share menu if tapped and held:

![](https://site.tld/image.png)  

However, an image written this way will just collapse the comment:

![a really cool image](https://site.tld/image.png)